Dry developers for use in the electrophotographic method and so forth are divided into two groups: one component developers in which there are used toners comprising a binder resin and a coloring agent dispersed therein; and two component developers in which the above toners are combined with carriers. In a copying operation using the developers, an electrostatic latent image formed on an electrophotographic photoreceptor, for example, is developed with the developer to from a toner image, which is then transferred, and toners remaining on the photoreceptor are removed by cleaning. Thus the dry developer is required to meet various requirements at the copying process, particularly a developing step and a cleaning step. Since the toners are applied to development not in the form of aggregated mass but in the form of divided particles, it is necessary that the toners have sufficient fluidity and are not changed in fluidity or electrical properties with a lapse of time or depending on conditions (temperature and humidity). In the case of the two component developer, it is necessary that the developer does not cause a so-called toner filming phenomenon that toners are firmly attached onto the surface of carriers. Moreover, in cleaning, it is required that the remaining toners can be easily removed from the surface of the photoreceptor, and that when a cleaning member such as a blade or a web is used, it does not damage the photoreceptor. In order to comply with these requirements, there have been various dry developers, one component developers or two component developers, in which inorganic fine powder such as silica, organic fine powder such as fatty acid and its derivative or metal salt, or fluorine-based resin fine powder is added to toners, so that fluidity, durability or cleaning properties are improved.
In connection with silica, it is known to use silica which has been made hydrophobic, and silica treated with an electric charge controlling substance as described in JP-A-55-135855 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Although hydrophobic silica is greatly effective in increasing powder fluidity, its great negative chargeability produces a hindrance for use in positively chargeable toners. Thus it is described in the aforementioned JP-A-55-135855 that the silica is treated with an electric charge controlling substance which makes the toners positively chargeable. However, since dye is used as the electric charge controlling substance, the silica is colored and thus cannot be applied to color toners.
Thus it has been desired to develop silica particles which can be applied to color toners and further which meet requirements such as powder fluidity, cleaning properties, and positive charge stability.